Check whether a modular subgroup is congruence from its generators

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I want to input the generators of a specific subgroup of the modular group $\Gamma = PSL(2,\mathbb{Z})$ as explicit $2\times 2$ matrices, and have Sage tell me whether that subgroup is congruence. Clearly I need to use the command is_congruence() for the second part, but I'm having trouble inputting the subgroup generators. Potentially it is something very simple - can anyone help? Thanks.